In the field of FA (Factory Automation), various devices have been controlled to share the process of work. Various controllers, remote I/O units and manufacturing devices are used for work in a certain area such as a factory facility. In order to operate these devices in conjunction with one another, an industrial network system which is also called a field network is constructed for connecting among these devices.
A typical industrial network system includes various slave devices for data collection from and control of production equipment installed in a factory, and a master device for centralized administration of these slave devices. In such an industrial network system, the slave devices and the master device are connected to communicate with one another, so that the process of production is controlled.
A network having a master device and slave devices may adopt various topologies, such as a serial topology, a ring topology, a tree topology and a star topology, in accordance with collaborative operations and wiring conditions between devices.
In the serial topology, all slave devices are on one transmission path formed with a master device defined as a starting point. It is assumed herein that the master device is located on the upstream side. On the transmission path, an information signal from the upstream side sequentially passes through the slave devices connected in series, and reaches the downmost stream-side slave device. Thereafter, the information signal is sent back from the downmost stream side, and is returned to the master device.
In the ring topology, a master device has two ports, i.e., a port for sending an information signal and a port for receiving an information signal. The sent information signal is sequentially passed through slave devices, and then is returned to the master device through the receiving-side port. In the serial topology and the ring topology, as described above, an information signal passes through one transmission path which is not branched.
In the tree topology or the star topology, on the other hand, a path from a master device is branched. A hub device is disposed as a network device on the branched portion. The hub device has one port connected to the upstream side, and a plurality of ports for connecting slave devices to the downstream side. Alternatively, a slave device is directly connected to different slave devices in some cases.
A technology called an industrial Ethernet utilizing the technology of the Ethernet (registered trademark) has been under development as one example of the industrial network system directed to the field of FA. The industrial Ethernet is also called an engineering Ethernet or a real-time Ethernet, and is such a network that Ethernet-related technologies and devices are introduced into the field of FA in various layers. With regard to the industrial Ethernet, various groups have established and released open standards. One example of the standards is the EtherCAT (Ethernet for Control Automation Technology: registered trademark) promoted by the ETG (EtherCAT Technology Group).
The EtherCAT standards also support the various topologies described above, and allow a combination of a serial connection like a chain with a branch using a hub device. According to the EtherCAT, moreover, it is not that an information signal from a master device is received at only a specific address, but a single signal is utilized by all slave devices. The information signal contains control data for each slave device. Therefore, each slave device reads data therefor from the information signal, rewrites the data if necessary, and then transfers the information signal to the downstream-side slave device. Accordingly, the information signal reaches the downmost stream side without remaining at a certain position in a network. Therefore, it becomes possible to realize high-speed communication without data collision.
As described above, the information signal according to the EtherCAT travels throughout the network along a so-called unicursal route. This characteristic also applies to a topology including a branched portion. That is, when an information signal from a master device reaches a hub device which is a branched portion, a predetermined port is selected from a plurality of ports. Then, the information signal is transmitted to a slave device connected to the selected port. In a case where different slave devices are connected to the downstream side of this slave device, the information signal is sequentially passed through the downstream-side slave devices. The information signal reaches the downmost stream-side slave device, and then is returned to the hub device. Subsequently, the information signal is transmitted to the slave device connected to the different port of the hub device.
Japanese Unexamined Patent Publication No. 2008-124791 (Patent Document 1) discloses a method of monitoring a network failure in order to detect erroneous cable connections and erroneous settings in the Ethernet. Japanese Unexamined Patent Publication No. 2010-034876 (Patent Document 2) discloses a method of detecting occurrence of failures on a relay device and a communication line in a network, and identifying a position of the failure. However, none of the patent documents disclose a method of detecting a connection abnormality in an industrial network system where a master device communicates with a slave device.